Compilation Optimization Guide
This guide provides an introduction to optimizing compiled code using safe, sane
CFLAGS and CXXFLAGS. It also as describes the theory behind optimizing in
general.
42012-04-27IntroductionWhat are CFLAGS and CXXFLAGS?

CFLAGS and CXXFLAGS are environment variables that are used to tell the GNU
Compiler Collection, gcc, what kinds of switches to use when compiling
source code. CFLAGS are for code written in C, while CXXFLAGS are for code
written in C++.

They can be used to decrease the amount of debug messages for a program,
increase error warning levels, and, of course, to optimize the code produced.
The GNU
gcc handbook maintains a complete list of available options and their
purposes.

How are they used?

CFLAGS and CXXFLAGS can be used in two ways. First, they can be used
per-program with Makefiles generated by automake.

However, this should not be done when installing packages found in the Portage
tree. Instead, set your CFLAGS and CXXFLAGS in /etc/portage/make.conf. This
way all packages will be compiled using the options you specify.

CFLAGS="-march=athlon64 -O2 -pipe"
CXXFLAGS="${CFLAGS}"

As you can see, CXXFLAGS is set to use all the options present in CFLAGS. This
is what you'll want almost without fail. You shouldn't ever need to specify
additional options in CXXFLAGS.

Misconceptions

While CFLAGS and CXXFLAGS can be very effective means of getting source code to
produce smaller and/or faster binaries, they can also impair the function of
your code, bloat its size, slow down its execution time, or even cause
compilation failures!

CFLAGS are not a magic bullet; they will not automatically make your system run
any faster or your binaries to take up less space on disk. Adding more and more
flags in an attempt to optimize (or "rice") your system is a sure recipe for
failure. There is a point at which you will reach diminishing returns.

Despite the bragging you'll find on the internet, aggressive CFLAGS and CXXFLAGS
are far more likely to harm your programs than do them any good. Keep in mind
that the reason the flags exist in the first place is because they are designed
to be used at specific places for specific purposes. Just because one particular
CFLAG is good for one bit of code doesn't mean that it is suited to compiling
everything you will ever install on your machine!

Ready?

Now that you're aware of some of the risks involved, let's take a look at some
sane, safe optimizations for your computer. These will hold you in good stead
and will endear you to developers the next time you report a problem on Bugzilla. (Developers will usually request
that you recompile a package with minimal CFLAGS to see if the problem persists.
Remember, aggressive flags can ruin code.)

OptimizingThe basics

The goal behind using CFLAGS and CXXFLAGS is to create code tailor-made to your
system; it should function perfectly while being lean and fast, if possible.
Sometimes these conditions are mutually exclusive, so we'll stick with
combinations known to work well. Ideally, they are the best available for any
CPU architecture. We'll mention the aggressive flags later so you know what to
look out for. We won't discuss every option listed on the gcc manual
(there are hundreds), but we'll cover the basic, most common flags.

Whenever you're not sure what a flag actually does, refer to the relevant
chapter of the gcc
manual. If you're still stumped, try Google, or check out the gccmailing lists.
-march

The first and most important option is -march. This tells the compiler
what code it should produce for your processor architecture (or
arch); it says that it should produce code for a certain kind of CPU.
Different CPUs have different capabilities, support different instruction sets,
and have different ways of executing code. The -march flag will instruct
the compiler to produce code specifically for your CPU, with all its
capabilities, features, instruction sets, quirks, and so on.

Even though the CHOST variable in /etc/portage/make.conf specifies the
general architecture used, -march should still be used so that programs
can be optimized for your specific processor. x86 and x86-64 CPUs (among others)
should make use of the -march flag.

What kind of CPU do you have? To find out, run the following command:

$ cat /proc/cpuinfo

Now let's see -march in action. This example is for an older Pentium III
chip:

CFLAGS="-march=pentium3"
CXXFLAGS="${CFLAGS}"

Here's another one for a 64-bit AMD CPU:

CFLAGS="-march=athlon64"
CXXFLAGS="${CFLAGS}"

If you still aren't sure what kind of CPU you have, you may just want to use
-march=native. When this flag is used, GCC will detect your processor and
automatically set appropriate flags for it. However, this should not be
used if you intend to compile packages for a different CPU!

So if you're compiling packages on one computer, but intend to run them on a
different computer (such as when using a fast computer to build for an older,
slower machine), then do not use -march=native. "Native" means
that the code produced will run only on that type of CPU. The
applications built with -march=native on an AMD Athlon 64 CPU will not be
able to run on an old VIA C3 CPU.

Also available are the -mtune and -mcpu flags. These flags are
normally only used when there is no available -march option; certain
processor architectures may require -mtune or even -mcpu.
Unfortunately, gcc's behavior isn't very consistent with how each flag
behaves from one architecture to the next.

On x86 and x86-64 CPUs, -march will generate code specifically for that
CPU using all its available instruction sets and the correct ABI; it will have
no backwards compatibility for older/different CPUs. If you don't need to
execute code on anything other than the system you're running Gentoo on,
continue to use -march. You should only consider using -mtune when
you need to generate code for older CPUs such as i386 and i486. -mtune
produces more generic code than -march; though it will tune code for a
certain CPU, it doesn't take into account available instruction sets and ABI.
Don't use -mcpu on x86 or x86-64 systems, as it is deprecated for those
arches.

Only non-x86/x86-64 CPUs (such as Sparc, Alpha, and PowerPC) may require
-mtune or -mcpu instead of -march. On these architectures,
-mtune/-mcpu will sometimes behave just like -march (on
x86/x86-64) . . . but with a different flag name. Again, gcc's behavior
and flag naming just isn't consistent across architectures, so be sure to check
the gccmanual
to determine which one you should use for your system.

For more suggested -march/-mtune/-mcpu settings, please
read chapter 5 of the appropriate Gentoo
Installation Handbook for your arch. Also, read the gcc manual's
list of architecture-specific
options, as well as more detailed explanations about the differences
between -march, -mcpu, and -mtune.
-O

Next up is the -O variable. This controls the overall level of
optimization. This makes the code compilation take somewhat more time, and can
take up much more memory, especially as you increase the level of optimization.

There are five -O settings: -O0, -O1, -O2,
-O3, and -Os. You should use only one of them in
/etc/portage/make.conf.

With the exception of -O0, the -O settings each activate several
additional flags, so be sure to read the gcc manual's chapter on optimization
options to learn which flags are activated at each -O level, as
well as some explanations as to what they do.

Let's examine each optimization level:

-O0: This level (that's the letter "O" followed by a zero) turns off
optimization entirely and is the default if no -O level is specified
in CFLAGS or CXXFLAGS. Your code will not be optimized; it's not normally
desired.

-O1: This is the most basic optimization level. The compiler will try
to produce faster, smaller code without taking much compilation time.
It's pretty basic, but it should get the job done all the time.

-O2: A step up from -O1. This is the recommended level
of optimization unless you have special needs. -O2 will activate a
few more flags in addition to the ones activated by -O1. With
-O2, the compiler will attempt to increase code performance without
compromising on size, and without taking too much compilation time.

-O3: This is the highest level of optimization possible, and also the
riskiest. It will take a longer time to compile your code with this option,
and in fact it should not be used system-wide with gcc 4.x.
The behavior of gcc has changed significantly since version 3.x. In
3.x, -O3 has been shown to lead to marginally faster execution times
over -O2, but this is no longer the case with gcc 4.x.
Compiling all your packages with -O3will result in larger
binaries that require more memory, and will significantly increase the odds
of compilation failure or unexpected program behavior (including errors).
The downsides outweigh the benefits; remember the principle of diminishing
returns. Using -O3 is not recommended for gcc 4.x.

-Os: This level will optimize your code for size. It activates all
-O2 options that don't increase the size of the generated code. It
can be useful for machines that have extremely limited disk storage space
and/or have CPUs with small cache sizes. However, it can cause quite a few
problems, which is why it is filtered out by many of the ebuilds in the
tree. Using -Os is not recommended.

As previously mentioned, -O2 is the recommended optimization level. If
package compilations error out, check to make sure that you aren't using
-O3. As a fallback option, try setting your CFLAGS and CXXFLAGS to a
lower optimization level, such as -O1 or even -O0 -g2 -ggdb (for
error reporting and checking for possible problems) and recompile the package.

-pipe

A common flag is -pipe. This flag actually has no effect on the
generated code, but it makes the compilation process faster. It tells the
compiler to use pipes instead of temporary files during the different stages of
compilation, which uses more memory. On systems with low memory, gcc might get
killed. In that case, do not use this flag.

-fomit-frame-pointer

This is a very common flag designed to reduce generated code size. It is turned
on at all levels of -O (except -O0) on architectures where doing
so does not interfere with debugging (such as x86-64), but you may need to
activate it yourself by adding it to your flags. Though the GNU gcc
manual does not specify all architectures it is turned on by using -O,
you will need to explicitly activate it on x86. However, using this flag will
make debugging hard to impossible.

In particular, it makes troubleshooting applications written in Java much
harder, though Java is not the only code affected by using this flag. So while
the flag can help, it also makes debugging harder; backtraces in particular will
be useless. However, if you don't plan to do much software debugging and
haven't added any other debugging-related CFLAGS such as -ggdb, then you
can try using -fomit-frame-pointer.

Do not combine -fomit-frame-pointer with the similar flag
-momit-leaf-frame-pointer. Using the latter flag is discouraged, as
-fomit-frame-pointer already does the job properly. Furthermore,
-momit-leaf-frame-pointer has been shown to negatively impact code
performance.
-msse, -msse2, -msse3, -mmmx, -m3dnow

These flags enable the SSE, SSE2, SSE3, MMX, and 3DNow! instruction sets for x86
and x86-64 architectures. These are useful primarily in multimedia, gaming, and
other floating point-intensive computing tasks, though they also contain several
other mathematical enhancements. These instruction sets are found in more modern
CPUs.

Be sure to check if your CPU supports these by running cat /proc/cpuinfo.
The output will include any supported additional instruction sets. Note that
pni is just a different name for SSE3.

You normally don't need to add any of these flags to /etc/portage/make.conf
as long as you are using the correct -march (for example,
-march=nocona implies -msse3). Some notable exceptions are newer
VIA and AMD64 CPUs that support instructions not implied by -march (such
as SSE3). For CPUs like these you'll need to enable additional flags where
appropriate after checking the output of cat /proc/cpuinfo.

You should check the list
of x86 and x86-64-specific flags to see which of these instruction sets are
activated by the proper CPU type flag. If an instruction is listed, then you
don't need to specify it; it will be turned on by using the proper -march
setting.
Optimization FAQsBut I get better performance with -funroll-loops -fomg-optimize!

No, you only think you do because someone has convinced you that more
flags are better. Aggressive flags will only hurt your applications when used
system-wide. Even the gccmanual
says that using -funroll-loops and -funroll-all-loops makes code
larger and run more slowly. Yet for some reason, these two flags, along with
-ffast-math, -fforce-mem, -fforce-addr, and similar flags,
continue to be very popular among ricers who want the biggest bragging rights.

The truth of the matter is that they are dangerously aggressive flags. Take a
good look around the Gentoo Forums
and Bugzilla to see what those flags
do: nothing good!

You don't need to use those flags globally in CFLAGS or CXXFLAGS. They will only
hurt performance. They may make you sound like you have a high-performance
system running on the bleeding edge, but they don't do anything but bloat your
code and get your bugs marked INVALID or WONTFIX.

You don't need dangerous flags like these. Don't use them. Stick to the
basics: -march, -O, and -pipe.

What about -O levels higher than 3?

Some users boast about even better performance obtained by using -O4,
-O9, and so on, but the reality is that -O levels higher than 3
have no effect. The compiler may accept CFLAGS like -O4, but it actually
doesn't do anything with them. It only performs the optimizations for
-O3, nothing more.

Oftentimes CFLAGS and CXXFLAGS that are turned on at various -O levels
are specified redundantly in /etc/portage/make.conf. Sometimes this is done
out of ignorance, but it is also done to avoid flag filtering or flag replacing.

Flag filtering/replacing is done in many of the ebuilds in the Portage tree. It
is usually done because packages fail to compile at certain -O levels, or
when the source code is too sensitive for any additional flags to be used. The
ebuild will either filter out some or all CFLAGS and CXXFLAGS, or it may replace
-O with a different level.

The Gentoo
Developer Manual outlines where and how flag filtering/replacing works.

It's possible to circumvent -O filtering by redundantly listing the flags
for a certain level, such as -O3, by doing things like:

CFLAGS="-O3 -finline-functions -funswitch-loops"

However, this is not a smart thing to do. CFLAGS are filtered for
a reason! When flags are filtered, it means that it is unsafe to build a package
with those flags. Clearly, it is not safe to compile your whole system
with -O3 if some of the flags turned on by that level will cause problems
with certain packages. Therefore, you shouldn't try to "outsmart" the developers
who maintain those packages. Trust the developers. Flag filtering and
replacing is done for your benefit! If an ebuild specifies alternative flags,
then don't try to get around it.

You will most likely continue to run into problems when you build a package with
unacceptable flags. When you report your troubles on Bugzilla, the flags you use
in /etc/portage/make.conf will be readily visible and you will be told to
recompile without those flags. Save yourself the trouble of recompiling by not
using redundant flags in the first place! Don't just automatically assume that
you know better than the developers.

What about LDFLAGS?

The Gentoo developers have already set basic, safe LDFLAGS in the base profiles,
so you don't need to change them.

Can I use per-package flags?
Using per-package flags complicates debugging and support. Make sure you mention
in your bug reports if you make use of this feature and what the changes are you
made.

Information on how to use per-package environment variables (including CFLAGS)
is described in the Gentoo
Handbook, "Per-Package Environment Variables".

Resources

The following resources are of some help in further understanding optimization:

The GNU gcc
manual

Chapter 5 of the Gentoo Installation
Handbooks

man make.conf

Wikipedia

Acovea, a
benchmarking and test suite that can be useful for determining how different
compiler flags interact and affect generated code, though its code
suggestions are not appropriate for system-wide use. It is available in
Portage: emerge acovea.